1. Field of the Invention
The present invention relates generally to a varifocal lens system, and more particularly, to a compact zooming lens assembly capable of macrophotography whereby extremely close-up photography can be accomplished, as well as, the normal zooming photography of an object positioned between infinity and relatively close positions to the lens system.
2. Description of the Prior Art
In the field of photography, zooming lens assemblies and macrophotography or close-up focusing assemblies are well known in the prior art. A zoom lens system that is further capable of macrophotography is disclosed in the Ogura et al, U.S. Pat. No. 3,912,376 assigned to the same assignee of the present invention. In that zoom lens system, a first front focusing lens group has a positive refractive power, while the zooming lens groups can be subdivided into positive and negative refractive powered subcomponents. To achieve macrophotography, one of the zooming lens group subcomponents can be held stationary while the other subcomponents can be moved along the optical axis to a close-up focusing position.
The Someya, U.S. Pat. No. 3,661,445, discloses a zoom lens system with macrophotography capabilities wherein a first focusing lens of positive refractive power is combined with a negative refractive power variator. In the macrophotography mode of operation, the variator lens group is held in a telephoto position and a positive refractive power lens group of the variator is shifted for focusing.
The Hirose et al, U.S. Pat. No. 3,659,921, discloses a zoom lens assembly capable of macrophotography wherein the focusing lens is shifted forward and the front group of a relay lens is shifted toward the image plane to provide the macrophotography mode of operation.
Another zoom lens assembly with macrophotography is provided in U.S. Pat. No. 3,817,600, granted to Watanabe et al., wherein a lens group in the zooming group and a lens group in the compensating group are moved in a predetermined relationship to provide a macrophotography mode of operation.
The Suzuki, U.S. Pat. No. 3,655,271 and Gela et al., U.S. Pat. No. 3,773,402, are cited of general interest to disclose varifocal lens systems with macrophotography capabilities.
The traditional design criterion for varifocal lens systems capable of macrophotography has been to provide a front lens component of positive refractive power to minimize optical design problems. A lens system, disclosed in Japanese patent application No. 68157/47, laid open to public inspection in Japan as File No. 29146/49, consists of only two lens groups with a negative front lens group and a positive rear lens group. This lens design is to provide a relatively limited capability and simple design. In each of the above cited prior art patents, the varifocal lens assembly includes generally a front focusing lens group which is positioned on the object side of the lens sustem and is of a convergent or positive refractive power. The lens systems also include a variator lens group and a compensator lens group that are generally movable for varying the focal length without a substantial image plane deviation from the focal plane. In addition, each of the above varifocal lens systems provide a particular mode of movement for at least one of the lens components to effectuate a focusing capability in the macrophotography range.
Finally, a varifocal objective lens system having a close focusing distance in the order of two to three times the absolute focal distance of the focusing group is disclosed in the Lai et al U.S. Pat. No. 3,603,669. The Lai et al varifocal lens system has a divergent front lens component, and a pair of movable lens groups filling the function of variator and compensator at the same time.
In the prior art varifocal lens systems mentioned above, the front lens component is usually movable on the optical axis for focusing an object between infinity and a relatively close position. When a convergent front lens component having positive refractive power is utilized, it must be moved a relatively large distance towards the object side of the lens system for focusing on a relatively close object. This movement creates unfavorable aberration problems for the optical designer. In addition, the diameter of a convergent front lens component must be relatively large to permit the introduction of a sufficient amount of light when the front lens component is moved forward to its extreme object side position. As can be appreciated, the diameter of the convergent front lens component must be accordingly increased if a wide angle varifocal lens system is desired. The relative axial displacement of the convergent front lens component along with the necessity of a relatively large diameter, creates a relatively large and bulky lens system.
The use of a divergent front lens focusing component can alleviate some of these aberration and size problems created by a convergent front lens component. However, heretofore, it has been generally difficult to design a varifocal lens system having adequate imaging properties with a divergent front lens component. In particular, a varifocal lens system that is capable of macrophotography has been thought to create special optical problems. To date, the prior art has not provided a varifocal lens system that is capable of macrophotography wherein a divergent front focusing lens component is combined with a variator component and a compensator component in a relatively compact lens assembly. The prior art has instead provided numerous examples of varifocal lens systems capable of macrophotography with a convergent front focusing lens component.